We investigated the importance of LHRH on the maintenance of FSH and LH immunoreactivity in gonadotrophs. Hypophysectomized orchidectomized hamsters (hosts) each received an allograft of a 7-week-old male hamster pituitary gland beneath their right renal capsule. Starting 6 days after transplantation, hosts were injected sc, twice daily with 1 micrograms LHRH or vehicle for 16 days. Twelve hosts in each group were killed by decapitation 16 h after the last injection. Allografts from six of the hamsters in each group and pituitary glands in situ from 10-week-old normal males were prepared for histological examination. Sections of tissue were stained for FSH or LH and with hematoxylin. Allografts from the remaining hamsters were homogenized to measure FSH and LH concentrations. In allografts from the vehicle-treated hosts, 22.8% of adenohypophysial cells stained for LH, while only 16.9% stained for FSH. In allografts from LHRH-treated hosts, 22.6% and 23.8% of the adenohypophyses cells stained for LH and FSH, respectively. Adenohypophyses that developed for the same length of time in situ had 24.8% and 24.1% of the cells staining for LH and FSH, respectively. Matching of some of the FSH and LH cells in serial flip-flopped sections of tissue from all hamsters revealed that many if not all gonadotrophs contained LH. LH- and FSH-containing cells in allografts were similar in size and shape, but were smaller and more circular in profile than those observed in situ. Treatment of hosts with LHRH did not alter gonadotroph size or shape, but it did reduce allograft LH concentration and elevate the serum FSH concentration compared to that in the vehicle-treated hamsters. These results suggest that in the hamster LHRH 1) plays a major role in maintaining FSH immunoreactivity in adenohypophysial tissue, 2) does not play a role in maintaining numbers of immunoreactive LH cells in adult adenohypophysial tissue, and 3) functions to maintain FSH synthesis at least in part in cells that contain LH.